An electrophotographic photoreceptor must have the functions of holding surface charge in dark environments, generating charge upon receiving light and also transporting charge upon receiving light, and there exist both single-layer receptors, which combine all these functions in one layer, and multilayer receptors, which consist of stacked layers with separate functions—primarily a layer that contributes to charge generation and a layer that contributes to holding surface charge in dark environments and transporting charge upon receiving light.
The Carlson method for example is applicable to image formation by electrophotography using these electrophotographic photoreceptors. In this system, image formation involves electrification by corona discharge and contact with the receptor in a dark environment, formation of the letters, pictures or other electrostatic images from a manuscript on the surface of the electrified photoreceptor, development of the electrostatic images by means of a toner, and transference and attachment of the developed toner image on a paper or other support. After transference of the toner image, the photoreceptor can be reused after residual toner removal and optical neutralization as necessary.
Conventionally, the photosensitive materials of these electrophotographic photoreceptors have consisted of inorganic photoconducting substance such as selenium, selenium alloys, zinc oxide, cadmium sulfide and the like dissolved and dispersed in resin binders, or of organic photoconducting materials such as poly-N-vinylcarbazole, 9,10-anthracenediol polyester, hydrazone, stilbene, benzidine, triphenylamine, phthalocyanine, bisazo compounds and the like dissolved and dispersed in resin binders, or of such photoconducting substance as vacuum deposits or sublimates.
Known resin binders for use in electrophotographic receptors include polyamide, epoxy resin, alkyd resin, poly aryl resin, polyvinyl chloride, polyvinyl acetate, polyketals, silicone resin, polystyrene, polymethyl methacrylate, polycarbonate and the like.
Of these, polystyrene is known to be cheap and economical, and electrophotographic receptors using polystyrene have been reported in various patent documents and the like. For example, Netherlands Patent No. 6410840 gives a typical example as a base material for forming a photosensitive layer. U.S. Pat. No. 3,926,626 gives a typical example as a soft insulating layer material for a photoreceptor. Japanese Patent Application Laid-open No. H5-165237 gives a general example of a resin binder consisting of polystyrene that is combined with X-form metal-free phthalocyanine and an oxazole compound, and then dissolved in a solvent.
Thus, polystyrene is generally known as a material for use in electrophotographic photoreceptors, but because photosensitive layers using polystyrene have an extremely large permanent deformation rate as shown in Itami et al., Konica Technical Report, (14), 43 (2001) (“Konica Technical Report”), they have poor wear resistance and are not used for practical applications. Instead, expensive resin binders such as polycarbonate and poly aryl are presently used.
In addition, mineral oil is sometimes added to polystyrene in order to improve fluidity and the like during use in molding applications.
Currently, however, various positive-charging multilayer electrophotographic photoreceptors are being reported in the field of electrophotographic photoreceptors, and for example Japanese Patent Application Laid-open No. S61-34547 reports on one consisting of a charge transport layer containing a charge transport material comprising an electron donating compound and a charge generating layer containing a specific crystalline metal-free phthalocyanine layered successively on an electroconductive layer. Japanese Patent Application Laid-open No. S61-48868 reports on forming a blocking layer containing a specific charge transport material under a single photosensitive layer containing a specific charge generating substance. Japanese Patent Application Laid-open No. S62-231262 reports on successively layering a charge transport layer containing a specific substance and a charge generating layer containing a specific substance, and gives polystyrene as one example of a binder resin for the charge transport layer. Finally, Japanese Patent Application Laid-open No. H4-242259 reports on an electrophotographic photoreceptor having a carrier transport layer and a carrier generating layer in that order on an electroconductive substrate, wherein the carrier generating layer contains a P-type carrier transport material and an N-type carrier transport material.
As discussed above, it is known that polystyrene is economical for use as a resin binder in electrophotographic photoreceptors, and that some electrophotographic photoreceptors are of the positive-charging multilayer type, but the wear resistance properties and durability of positive-charging multilayer electrophotographic photoreceptors may not be satisfactory when polystyrene is used as the resin binder.